This invention relates to fiber optic devices. In particular, the invention relates to a fiber optic connecting cable that is coiled to provide for increasing and decreasing the length of the cable.
There are many applications in which it is useful to have a fiber optic cable connecting to fiber optic devices that are movable with respect to one another. This may be true, for example, with a robot that is controlled by fiber optics, in which case it may be desirable to connect a moving part of the robot to another moving part or to a fixed location. It is also useful to be able to interconnect items of fiber optic laboratory or test equipment by cables that eliminate the need for precise spacing of the equipment. However, when conventional fiber optic cables are used for these or other similar applications, the use of enough cable to minimize the risk of breakage in a position of extreme separation is likely to lead to a cable that will sag when the interconnected parts or equipment are placed are closer together. It would be of great utility to have a fiber optic cable that would assume a range of lengths without sagging and have good stability and strength.
Two basic modes of transmission of optical information are used in optical fibers: single-mode and multi-mode. Single-mode transmission of light in an optical fiber is used when it is necessary to preserve phase information of the light. The term "single-mode" refers to the fact that little or no reflection of light within the fibers is permitted. In multi-mode transmission, on the other hand, where relative phase information is not important, the principal means of light transmission through a fiber is by multiple internal reflections from the walls of the fiber. Both single-mode and multi-mode fibers are normally produced with an internal fiber of either silica glass or a transparent plastic of a uniform index of refraction. Either kind of fiber is coated in the process of manufacture with a material that has an index of refraction that is different from and generally less than that of the main conducting fiber. If the index of refraction of the fiber is uniform, this is referred to as a step-index cladding. If the index of refraction of the fiber varies continuously from a maximum value on and near the axis to the value of the cladding at their interfaces, this is referred to as a graded index fiber. The cladding causes light that is directed at an angle to the axis of the fiber to be refracted back toward the axis.
In making fiber optic cables in the past, much effort has gone into protecting the optical fibers from breaking due to tension or kinking. To protect the fibers in tension, methods have been used such as supplying steel as a tension member and winding the optical fiber about the steel. This tends to lead to a cable that is relatively inflexible and disproportionately heavy for use in supplying optical connection to a moving part.
Therefore, there is a need for a fiber optic connecting cable that is relatively lightweight, is coiled, and can be stretched without objectable sagging and compressed without unwanted breakage of the optic fiber or fibers.